From my perspective, I looked at class Cs - 2000 and back:
Either choice is going to require work - especially at that price range. How much and where the work is needed is dependent upon the individual RV - mileage, age, brand, etc., will really have nothing to do with what has gone/will go wrong. MPG won't be that much different between the two (unless engine issues have affected one).
Get a quality mechanic to check out both engines. Test all appliances (stove, fridge, heater, water heater, etc.) on all systems (propane, electric, 12v). Go with the RV with the best engine quality (don't want to be broken down on the side of the road).
Appliances going bad can be worked around until you can afford to get them fixed. For example, if the stove goes out, get a camp stove to bide you over; if the heater goes out, get a Mr. Buddy.; and so on (if you'll have electrical hookups, then you can use microwave, electric frypan, electric heater, etc.). If the engine goes bad - you either fix it now or you're stranded.
Water leaks - yes, old RVs are subject to water leaks. Check it out thoroughly for major damage and plan on sealing every seam before the next rainy season. Each season thereafter, check for cracks, holes in the sealing before the first rain, and check for any leaks after the first rain - at the first sign of either, re-seal.
Get all rubber checked out (fuel lines, belts, hoses, etc.), including propane lines for leaks. Plan on replacing tires asap.
I have a 75 American Clipper. I LOVE it and wouldn't trade it for the world. If I had come here before buying it, I very likely would have been scared off by the "it's too old". Yes, I've had to do work to it - engine work mostly. The fridge is original and (knock on wood) still works great! The heater had a propane leak - I had the propane shut off from it and don't use it, I can repair later if I choose to. The stove/oven is original and (knock on wood) still works great! The water heater was replaced at some point before I bought the RV and (knock on wood) works great. Battery charger died (but converter portion works fine), so I had a PD charger/converter added and hooked up only the charger side (why pay for the extra re-wiring at that point). At this point, it runs and works great (knock on wood). I LOVE my RV.
1975 American Clipper RV with Dodge 360 (photo in profile)
1998 American Clipper Fold n Roll Folding Trailer
Both born in Morgan Hill, CA to Irv Perch (Daddy of the Aristocrat trailers)